TCI - Update CONTRIBUTING to include build information

Change-Id: I0423236d771284764e47214610ec860c0c12b453
Signed-off-by: Eugen Neufeld <eneufeld@eclipsesource.com>
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 55a835f..e948082 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -27,6 +27,9 @@
 * http://git.eclipse.org/c/emfclient/org.eclipse.emf.ecp.other.git
 * http://git.eclipse.org/c/emfclient/org.eclipse.emf.ecp.releng.git
 
+You will only need the http://git.eclipse.org/c/emfclient/org.eclipse.emf.ecp.core.git repository.
+In order to build you need to trigger `mvn clean install` on `releng/org.eclipse.emf.ecp.releng/pom.xml`.
+
 This project uses Bugzilla to track ongoing development and issues.
 
 * Search for issues: https://eclipse.org/bugs/buglist.cgi?product=ECP